The resistance and longevity of timepiece springs, particularly mainsprings, is a long-standing problem. Timepiece spring manufacturers are always looking for materials providing an increased service life, essentially with improved fatigue resistance, and an increased power reserve for accumulator springs, mainsprings or striking springs in particular.
The use of high carbon steels very quickly allowed the desired characteristics of elasticity to be obtained, but their sensitivity to corrosion, combined with permanent use under forces closed to their fracture load, has frequently resulted in fracturing as soon as corrosion spots appear. Further, these steels tend to have permanent deformations, which impair the power reserve, since their proportional maximum lengthening is much lower than their limit of elasticity.
Numerous alloys have been tested, in the most varied compositions and with various treatments. Patent Nos. BE475783, CH279670 and U.S. Pat. No. 2,524,660 in the name of Elgin propose solutions employing a cobalt based alloy, a combination of chromium-molybdenum, and a combination of nickel, iron and manganese, with complex production methods which increase the cost of the product.
WO Patent No 2005/045532, in the name of Seiko, proposes a titanium based alloy, supplemented with vanadium group elements.
Some manufacturers have developed springs with surface layers different from the core material, such as in WO Patent No. 02/04836 in the name of Seiko, or CH Patent No 383886 in the name of Sandvik, or CH Patent No 330555 in the name of Fabrique Suisse de Ressorts d'Horlogerie, or EP Patent No 2511229 in the name of GFD-Diamaze, or EP Patent No 1422436 in the name of CSEM.
Amorphous alloys are also known from WO Patent No 2012/01941 in the name of Rolex, with a high proportion of boron, or EP Patent No 2133756 in the name of Rolex (metallic glass), or from DE Patent No 102011001783 in the name of Vacuumschmelze.
All these materials are extremely expensive, and no product that is really more effective than others for the application concerned has appeared on the market.
Numerous wholesale alloys could, purely theoretically, be suitable for manufacturing timepiece springs, but experimental testing of these alloys in real production conditions encounters numerous limitations, which explains the very limited development with respect to materials used in the watch industry for manufacturing springs, especially spiral springs.
Consequently, a large number of alloys, which would, on paper, be suitable, and which are perhaps suitable for macromechanics, electrical engineering, heavy machinery or suchlike, prove totally unworkable, as soon as attempts are made to convert them to the dimensions required for watchmaking.
There is known from CH Patent No 703796 in the name of Générale Ressorts, a nitrogen stainless steel including a base formed of iron and chromium, arranged in a face centred cubic austenitic structure. The alloy described in this document has a high concentration of nitrogen in solution (0.75 to 1%). During manufacture of the alloy, the concentration of nitrogen in the solution is difficult to control in a precise manner. A low increase in the amount of nitrogen in solution in the alloy may lead to a loss of ductility of the alloy, which defeats the required purpose of a material to be used as a spring.
Further, the nitrogen content has a strong influence on the precipitation kinetics of chromium nitrides, and when the nitrogen content is around 1%, the speed of tempering of the alloy which prevents the appearance of nitrides is high, which makes it difficult and expensive to industrialise the treatment processes for these alloys.
Further, the manufacture of springs from these alloys is very problematic. The conventional production plan consists in transforming a cast billet of alloy by forging, rolling, then processing by drawing or wire drawing a wire rod having a diameter of around 6 mm, which is then skinned and cleaned, prior to a series of cold rolling and wire drawing operations: in particular, the skinning and wire drawing operations are especially difficult, or impossible when it is sought to obtain springs of very small dimensions, particular spiral mainsprings for timepieces having a thickness of less than 0.200 mm, or balance springs for an escapement mechanism which may have a thickness of around 0.050 mm.
Indeed, these operations, which are necessarily carried out on the material, result in significant temperature elevations, of several tens or hundreds of degrees Celsius. Nitrogen steels, with a nitrogen content of around 1% or more, are very sensitive to such temperature elevations, since, from around 200″C., precipitations of nitrides or other embrittling compounds may be produced, which prohibits any watchmaking application for alloys whose theoretical composition ought to be satisfactory in order to achieve the required characteristics of elasticity. Embrittlement produces cracking in the drawn wire, making it unsuitable for secondary operations.
A reduction in the rolling and wire drawing speeds may reduce but not eliminate these temperature elevations, but these speeds are then so low that the cost of the material becomes prohibitive for industrial use. Indeed, to change from a diameter of 6 mm to a diameter of around 0.6 mm (i.e. in a cross-sectional area ratio of 100:1), between 30 and 50 successive wire drawing operations must be carried out (assuming that the cross-section is reduced by 9 to 15% each time), and more accurately around 50 operations in order to limit the number of heating points, in addition to the intermediate heat treatments which are also necessary.
Nitrogen steels are difficult to produce, difficult and expensive to implement, and consequently, they have met with little enthusiasm in the field of precision or ordinary mechanical engineering, the only known fields of application being orthodontics, prosthetics and electrotechnics (retaining rings for motors or alternators), hence essentially macroscopic or heavy machinery applications. The theoretical specific qualities attributed to nitrogen steels thus clash with practical realisation.
It is therefore not possible to use any type of nitrogen steel for manufacturing timepiece springs, because of these drawbacks, and it is important to make a very specific selection in order to produce a material, used as raw wire material, typically having a diameter of around 0.60 to 1.00 mm, which is then transformed by cold rolling to obtain a spring of substantially rectangular section.
The problem for the timepiece spring manufacturer is thus to determine an alloy having suitable nitrogen and carbon content to make it possible to produce, first a raw wire material having a diameter of several tens of millimeters, and then a profiled spring having a substantially rectangular section and a thickness of several hundredths of a millimeter.
Although an evident peculiarity of timepiece springs is their particular dimensions, another feature consists in their employment in very specific conditions of metal fatigue: these springs are permanently subjected to forces close to their fracture limit, which is known as oligocyclic fatigue. A material working at oligocyclic fatigue must be particularly perfect, to prevent any premature fracture after a reduced number of cycles.
An examination of alloys which, in theory, could be suitable for the manufacture of timepiece springs will logically concern austenitic alloys with a face centred cubic structure.
U.S. Pat. No. 6,682,582B1 in the name of Speidel BASF describes various alloys, with a high proportion of chromium (16 to 22%), between 0.08% and 0.30% by mass of carbon, and between 0.30% and 0.70% by mass of nitrogen, and less than 9% manganese and less than 2% molybdenum.
KR Patent No 2009 0092144 in the name of Korea Mach. & Materials INST discloses a manganese-chromium-nickel-molybdenum alloy with the total content of carbon and nitrogen comprised between 0.60% and 0.90% by mass, with notably, in some alloys of the family having a carbon content of less than 0.45% by mass and a nitrogen content of less than 0.45% by mass.
JP Patent No H02156047 in the name of Nippon Steel Corp discloses an alloy with 5 to 25% manganese, 15 to 22% chromium, 0.10% to 0.30% carbon and 0.3% to 0.6% nitrogen.
Choosing an alloy that can actually be transformed to manufacture a timepiece spring is difficult, faced with the wealth of literature. A large number of documents describe alloys, which, only in theory, could be suitable, since they are austenitic alloys which seem to have the required peculiarities, such as as JP Patent Application No 2004137600A in the name of Nano Gijutsu Kenkyusho, JP Patent Application No 2009249658A in the name of Daido Steel Co Ltd., FR Patent Application No 2776306A1 in the name of Ugine Savoie SA, or DE Patent Application No 19607828A1 in the name of VSG EN & Schmiedetechnik GmbH.
It is clear that, although all the alloys described in these documents could in theory be suitable, very few satisfy the shaping requirements of those skilled in the art, who must then undertake extensive testing in order to make a selection, and test each selected alloy in real production conditions, which is not within the grasp of the mere reader of these documents.
More specifically, a mainspring, the drive element of a mechanical watch, is manufactured from a metal strip, and then wound around an arbour and housed inside a barrel drum. The document by Aurèle MAIRE, in the Journal Suisse d'horlogerie, vol. 5/6, 1 Jan. 1968, pages 213-214 XP001441388, sets out a theory of fast-rotating barrels, describing the free, treble clef shape of a spiral spring, and the optimised geometry for maximum available energy.
The conventional manufacture of a spiral spring, particularly a mainspring, from a raw wire material having a diameter of several tens of mm (which is already a product transformed during an extremely long and complex process as described above) is achieved in several steps:                rolling a metal wire to obtain a strip,        cutting the strip to a defined length, optionally also cutting out an aperture at one end thereof,        forming an eye at the end of the strip containing the aperture to enable the strip to be fixed to an arbour (either through an aperture made in the strip if the arbour contains a hook, or by the friction of the strip on the arbour). This step is carried out in two parts:                    forming the first eye which is a circle having a smaller diameter than the arbour to ensure that the hook is hooked in the aperture or the strip is held by friction, depending on the case;            forming a second eye which, in practice, is a spiral of increasing radius over around 0.75 turns, so as to ensure the eye is centred in the drum when the spring is let down.                        roller levelling the remainder of the strip in the opposite direction to the eye;        fixing the flange;        placing inside the drum.        
The peculiarity of the mainspring is that the material works at its maximum stress throughout the curvilinear abscissa due to the deformation imparted during the first winding. If the spring is removed from the drum, a treble clef shape of equilibrium results from the first winding.
For the watch designer seeking to produce springs with good resistance and satisfactory longevity, which can be produced in a reliable and especially repetitive manner, the difficulty lies in choosing or developing an alloy which enables the required performance to be obtained and can produce spiral springs including at least one area of thickness of less than 0.200 m, and/or including at least one area having a radius of curvature of less than 2.15 mm and notably less than 0.75 mm, or even 0.60 mm. The watch designer therefore cannot simply choose an alloy from a catalogue based on its theoretical physical characteristics, but must test a specific range of secondary operations, on the one hand for the wire serving as raw material, and on the other hand, for the finished spring, and set parameters for the composition and treatment of the alloy, which make it possible to produce wire blanks and springs of this type.